1. Field of the Invention
This invention relates to the treatment of coal and similar carbonaceous solids in the presence of alkali metal-containing catalysts and is particularly concerned with the recovery of alkali metal constituents from spent solids produced during coal gasification and similar operations.
2. Description of the Prior Art
Potassium carbonate, cesium carbonate and other alkali metal compounds have been recognized as useful catalysts for the gasification of coal and similar carbonaceous solids. The use of such compounds in coal liquefaction, coal carbonization, coal combustion and related processes has also been proposed. To secure the higher reaction rates made possible by the presence of the alkali metal compounds, it has been suggested that bituminous coal, subbituminous coal, lignite, petroleum coke, oil shale, organic wastes, and similar carbonaceous materials be mixed or impregnated with potassium, cesium, sodium or lithium compounds, alone or in combination with other metallic constituents, before such materials are reacted with steam, hydrogen, oxygen or other agents at elevated temperatures to produce gaseous effluents. Studies have shown that a wide variety of different alkali metal compositions can be used for this purpose, including both organic and inorganic salts, oxides, hydroxides and the like. Because of their relatively low cost and ready availability, inorganic salts such as potassium carbonate, cesium carbonate and mixtures of these and similar materials with other metallic compounds are generally preferred.
Coal gasification processes and similar operations carried out in the presence of alkali metal compounds at temperatures below about 2200.degree. F. generally result in the formation of chars and alkali metal catalyst residues. The chars normally include unconverted constituents of the coal or other feed material and various inorganic constituents generally referred to as ash. The amount of ash which is present depends in part on the composition of the feed material, the gasification or other reaction conditions employed, and the residence time of the particles within the reaction zone. It is generally advisable to withdraw a portion of the char from the reaction zone during gasification operations in order to eliminate the ash and keep it from building up within the gasifier or other vessels in the system. Elutriation methods and other techniques for separating char particles of relatively high ash content and returning particles of relatively low ash content to the reaction zone in order to improve utilization of carbon in such processes have been suggested.
There have been proposals for the recovery of alkali metal catalyst residues withdrawn from the reaction zone with the char during operations of the type referred to above by leaching out the alkali metal constituents. Studies indicate that these constituents are generally present in part as carbonates or other water-soluble compounds which can be recovered by water washing. More recent work has shown, however, that some of the alkali metal constituents may also be present in the char as insoluble compounds which are not readily susceptible to recovery by such techniques. Alkali metal compounds have been used as catalysts in coal gasification and similar operations to only a limited extent and hence relatively little work has been done in this area in the past.